Immunotherapy has emerged as a promising method to treat several cancers. In 2012 the FDA approved a new immunotherapy agent, ipilimumab, for metastatic melanoma. Ipilimumab activates T-cells and globally increases the adaptive immune response and results in an enhanced response to the cancer. In a subset of patients, this leads to longer Progression Free Survival (PFS), and in some cases remission. However, there is no companion diagnostic available to predict responders. The goal of this project is to develop a deep sequencing assay to measure the immune repertoire and a corresponding diagnostic protocol that differentiates ipilimumab responders from non-responders. Currently, while many of patients experience immune mediated side effects related to ipilimumab, less than 10% of patients cancer responds to therapy. Developing a companion diagnostic that differentiates responders from non-responders would protect non-responders from unnecessary side-effects.